Elimination of hydrogen from heats of killed steel



with an insulating material.

Patented Apr. 10, 1 951 1 ELIMINATION OF HYDROGEN FROM HEATS OF KILLEDSTEEL Charles R. Taylor and Charles F. Staley, Middletown, Ohio,assignors to Armco Steel Corporation, a corporation of Ohio No Drawing.Application January 12, 1946,

Serial No. 640,996

It is well known that hydrogen in killed steel is a source of blistersand flakes in thefinal rolled product, and results in the rejection oflarge quantities of valuable material. This is especially the case withstainless andotherchromium steels.

The primary object of this invention is the provision of a mode ofeliminating hydrogen or re-..

ducing it to that amount which the-solidified steel can hold insolution, so as to eliminate or minimize blistering from this source,easily, quickly, safely and economically. We accomplish this object andothers which will be set forth hereinafter-or will be apparent to oneskilled in the art upon reading these specifications, by that procedureof which we shall now describe an exemplary embodiment.

The actual amount of hydrogen which stainless steel, for example, canhold in solution just below its melting point, has not hitherto beenknown or published, so far as we are aware, Our researches haveindicated that this amount is in the, neighborhood of- .0007% by weight.If a molten bath of stainless steel contains more than this amount ofhydrogen, the excess above the solubility limit will be evolved duringsolidification of the metal, and will result in abnormally low'shrinkage, indicative of internal cavities which form blisters in thefinalrolled' metal; Conditions in the bath productive of abnormally lowshrinkage in the ingot canoccur at any time during the heat; but theseconditions usually occur after a reducing slag which is non-conductive,the ac,-

tion ofthe arcs is to blow the, slag away; from the surface of the metalabout the arcs, exposing itfor the absorption ofhydrogen. This hydrogenmay arise from moisture in the, air, but on ,oc casion there maybea'leak in the glands of thewater cooling jackets about the electrodes,permitting water to be delivered to the atmosphere inside the furnaceand sometimes to, drip upon; themetal itself.

'liher es n o ro en in.- e: bath y ea ly d c dw at; w rm a h i k test.In performing such a test a small quantity of the molten metal is merelytaken fromthe bath, poured into a small test mold, and covered When themetal 6 Claims. (ems-12) freezes its shrinkage characteristics areimmediately apparent. If the metal does not contain an excess ofhydrogen it will shrink in volume about 3%, which will show up as avisible cavity in the extreme top of the test piece. If the metalcontains an excess of hydrogen, the cavity will be abnormally small orthe surface may even risev in the mold, and the cross section of thecast Such shrink tests,

piece will show blow holes. performed at intervals throughout a heat,can be employed as a guide both to the duration of the treatmenthereinafter outlined, and to that point or points, in the course of aheat, at which the treatment may be applied.

' In the practice of our invention, we have found that if air, which hasbeen thoroughly dried, is blown through the bath, it will removecontained hydrogen in an amount depending upon the quantity of airblown. As an indication of what is meant by thoroughly dried, wehave'found that air dried so as to have a dew point at approximately 20 belowzero F. or lower is entirely satisfactory for our purpose.-

As an example of procedure under our inven tion, in producing stainlesssteel in the electric furnace, we blow the thoroughly dried air at approximately ten pounds pressure per square inch through a gas pipe in.in diameter'thrust through a furnace opening and immersed into the bath.The gas pipe is of ordinary iron, wrought iron or steel, and of astandard length, such as 15 feet. The, submergence of the pipe in thebath causes it to melt away progressively so that it is necessary tokeep thrusting it further and, further into the bath. When one such pipeis no longer usable, another is employed, andso on until the requiredamount of air is blown through the bath. More than one pipe may bethrust into the bath at a time if desired.

The consumption of pipe in this fashion fur: nishes a fair measureof theair blownat a certain pressure. In commercial practice we have foundthat it usually requires about. five pipesf of air to lowerthe hydrogencontent of a 10-ton heat to a value such that ingot shrinkage, will besatisfactory. More or fewer pipes may be blown as indicated by the,shrink test, It will be evident that the quantity of air blown may beotherwise measured, especially where an insulated conduit-or othernon-melting means is used for introducing the air into the bath. Thepressure, suggested is not, limitative, and may be considerablyvaried,rbut, isa good and safeoperating,

Theuse of'itoo great" 3 tering in the bath, which it is desirable toavoid. It is especially desirable to avoid such turbulence in the bathas would result in the splashing of molten metal onto the raised arcelectrodes, which metal, dripping back into the bath after having pickedup carbon from the electrodes, might harmfully afiect the carbon contentof the melt.

The blowing of thoroughly dried air through the bath as herein set forthis effective in removing hydrogen, but does not significantly alter thebath characteristics in any other way if performed while the bath isunder reducing conditions. As will be readily understood, reducingconditions are attained when the bath contains silicon or some otheractive deoxidizer or when the slag above the bath is high in silicon orits equivalent. Those skilled in the art will understand that it isusual in the production of chromium bearing steels to carry on thelatter stages of the heat under a slag to which ferrosilicon or the likeis added to return as much chromium as possible to the bath. When theblowing is done under reducing conditions, it produces negligibleoxidation or decarburization, and causes no lack of cleanliness in thesteel.

It is important that our treatment may be practiced when the bathcontains a large part or all of the desired chromium content, providingsilicon is also present. Under usual operating conditions the bath willcontain substantially .10 to .30 silicon. The silicon is removed to aslight extent depending upon the duration of the blow; but it protectsthe chromium so that the latter is not oxidized to an appreciabledegree. Hence, our treatment may be practiced, if desired, assubstantially the last treatment to which the heat is subjected prior totapping or pouring, or it may be practiced at any preceding stage atwhich its effect is useful. The treatment may also be repeated duringthe course of any heat, as shown to be desirable by test.

When the steel is formed or treated in the electric arc furnace, we stopthe arc and raise the electrodes during the blow. It is not necessary toremove the slag, although this may be done with advantage under certaincircumstances. The slag does not affect the removal of hydrogen by theblowing process; but hydrogen is lost, as we believe, by diffusioneither into the air blown through the bath or into the atmosphere abovethe bath, the latter being an action which can occur effectively only inthe absence of slag. Hence, removal of the slag will to some extentincrease the quantity of hydrogen removed in a blowing process of givenduration.

It will be understood that the air employed in our process may be driedin various ways and the particular manner of moisture removal is not alimitation on the invention. In practice we prefer to employ a drierhaving a chamber through which the air is passed and which containscalcium chloride or activated alumina as a drying agent, the latterusually being capable of lowering the moisture content further than theformer.

Modifications may be made in our invention without departing from thespirit of it. Having thus described our invention in an exemplaryembodiment, what we claim as new and desire to secure by Letters Patentis:

1. A process of producing commercially clean chromium-bearing steels ofthe stainless type, including the steps of producing killed steel in anelectric arc furnace, thoroughly deoxidizing the steel and during thefinishing stage and close enough to the tapping to prevent hydrogenreabsorption, passing dried atmospheric air through the steel in saidfurnace under a pressure insufficient to produce excessive splashing tothe extent of reducing the hydrogen in the steel to a value below itssolubility in the solidifled metal, and at a time when the steelcontains residual deoxidizing agent, the steel being finished under areducing slag.

2. The process claimed in claim 1 wherein the quantity of air so passedthrough the baths is sufficient substantially to remove by diffusioncontained hydrogen in excess of that soluble in the solidified steel, asdetermined by a shrink test.

3. The process claimed in claim 1 wherein the quantity of air so passedthrough the baths is sufiicient substantially to remove by diffusioncontained hydrogen in excess of that soluble in the solidified steel, asdetermined by a shrink test, the air being dried so as to have a dewpoint not substantially higher than 20 F. below zero.

4. The process claimed in claim 1 wherein the said deoxidizing agent issilicon and wherein the quantity of said silicon present in the steel isat least substantially .10% of the steel mass.

5. The process claimed in claim 1 wherein the treatment with atmosphericair is carried on while the steel is under the finishing slag.

6. In a process of producing commercially clean chromium-bearing steelof the stainless type in the electric arc furnace, the steps of meltingsteel forming ingredients and adding chromium under the influence ofelectric heat from the arc electrodes in the furnace, addingferro-silicon to displace chromium from a slag formed above said steel,thoroughly deoxidizing the steel by means of silicon with the provisionof residual silicon in the steel, raising said electrodes and withoutremoving said slag blowing through the molten steel atmospheric airdried.

so as to have a dew point not substantially in excess of 20 below zeroF. under low pressure whereby to reduce the hydrogen content of saidsteel without producing significant oxidation or decarburization to avalue not substantially in excess of the solubility of hydrogen insolidified steel, and thereafter tapping the steel from said furnacepromptly to prevent hydrogen re-absorption.

CHARLES R. TAYLOR. CHARLES F. STALEY.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,057,078 Morehead Mar. 25, 19131,641,326 Farnsworth Sept. 6, 1927 1,704,385 Hadfield Mar. 5, 19291,954,400 Arness Apr. 10, 1934 OTHER REFERENCES British Iron and SteelInstitute: 6th Report on the Heterogeneity of Steel Ingots, SectionVIII, page 147 and Figure 44 (1935) and 7th Report on the Heterogeneityof Steel Ingots, Section V, pages 139-142 and Plates 21 and 22 (1937).

1. A PROCESS OF PRODUCING COMMERCIALLY CLEAN CHROMIUM-BEARING STEELS OFTHE STAINLESS TYPE, INCLUDING THE STEPS OF PRODUCING KILLED STEEL IN ANELECTRIC ARC FURNACE, THROUGHLY DEOXIDIZING THE STEEL AND DURING THEFINISHED STAGE AND CLOSE ENOUGH TO THE TAPPING TO PREVENT HYDROGENREABSORPTION, PASSING DRIED ATMOSPHERIC AIR THROUGH THE STEEL IN SAIDFURNACE UNDER A PRESSURE INSUFFICIENT TO PRODUCE EXCESSIVE SPLASHING TOTHE EXTENT OF REDUCING THE HYDROGEN IN THE STEEL TO A VALUE BELOW ITSSOLUBILITY IN THE SOLIDIFIED METAL, AND AT A TIME WHEN THE STEELCONTAINS RESIDUAL DEOXIDIZING AGENT, THE STEEL BEING FINISHED UNDER AREDUCING SLAG.